Hindered Nucleophilic Displacement (S_N 2) Reactions of 2exo‐ and 2endo‐ Norbornyl Derivatives. Norbornane) Series 4

Abstract

SummaryThe reactions of 2exo-and 2endo-norbornyl bromide ( l e and 2e, respectively) in 90% ethanol with a large excess of potassium hydroxide, and of 2exo-norbornyl p-toluenesulfonate (1 g) with excess sodium thiophenolate in methyl cellosolve, have been studied. They obey the first order rate law and are zero order with respect to the base-nucleophile. However, the ratio of 1,2-and 1,3-elimi n~r i o n to exo-substitution products depends strongly on the base-nucleophile conci'n I I'LLtion. Ion pair intermediates are indicated. The extreme inertness of 2-norbornyl derivatives in displacement reactions is due to severe steric hindrance of rearside nucleophilic attack, a feature they have in common with other bi-and tricyclic molecules bearing nucleofugal groups adjacent to one or two bridgehead atoms.The literature contains surprisingly little evidence for bimolecular nucleophilic displacement reactions at C (2) of 2exo-and 2endo-norbornane derivatives') 1 and 2, respectively, when R represents a nucleofugal group. Claims that such reactions occur are based mainly on the formation of inverted products by reaction with strong nucleophiles under non-solvolytic conditions. Thus, 2exo-and Zendo-norbornyl p-bromobenzenesulfonate (1 a and 2 a, respec-I )

ChemInform Abstract: NORBORNANES. PART 8. POLAR VERSUS STERIC EFFECTS IN THE SOLVOLYSIS OF 6‐ENDO‐SUBSTITUTED 2‐ENDO‐NORBORNYL P‐TOLUENESULFONATES

“…There is, however, a small configurational effect since 6exo-substituents lead to somewhat higher rates than 6endo-substituents, as the k3/k6 ratios in Table 3 show7). A similar configurational effect of the substituent at C(6) was observed for the sulfonates 1 and 2, where k,lk, was also slightly larger thqn 1 [2]. It was there- Identified as the 'nortricyclanol' 18. d, Unpublished results. ")…”

“…A C (6) -, C (2) endo-hydride shift converts 28 to the tertiary carbenium ions 26, the precursors of 9-11. In contrast, the products 8 and 13 from the sulfonates 6d-6g are derived mainly from the unrearranged cations 27, confirming that -I-substituents retard rearrangement [2]. It is noteworthy that the same mixture of products should arise from the stereoisomeric p-toluenesulfonates 3 and 6 if the resultant cations were free and rearranged faster than they reacted with the solvent.…”

“…The correlation of the points for 6a-6g in the plot (Fig. ) is not as good as the ones for 1-3 [1] [2]. This is not surprising since the rate constants for 6a-6f were extrapolated to 140" in order to include the constant for the much less reactive cyano sulfonate 6g.…”

“…as the electron-attracting power of R increases. The large k,lk, ratio for R=OCOCH, of 337 is due to the accelerative anchimeric effect of this nucleophilic substituent in the sulfonate 2d which leads to cyclization to the cation 15 [2]. The enhanced kz/k6 ratio of 59 for R = OCH, is probably due to the accelerative hyperconjugative effect of this n-electron donor which, for stereoelectronic reasons, is more effective in 2i than in 6i (see below).…”

“…Furthermore, secondary steric effects, such as steric hindrance to solvation, are also bound to play a role. The reaction constant p for 6 is -0.94 and hence somewhat larger than for the 2endo-6exo-series 3 (p = -0.78), but considerably smaller than for the 2exo-6exo-series 1 @= -2.0) [l] and the 2exo-6endo-series 2 0, = -1.75) [2].…”